How do RED determine SD speed-worthiness?

I've read that RED state that most larger capacity SD cards do not meet the manufacturers claimed Read/Write speeds.
Does anybody know how this is detremined, apart from simply putting the SD card in the camera and trying to record onto it?
If that's all they do, I can't help wondering whether the problem might be with their camera, and that some cards are simply more tolerant of the interface.
I was also wondering if you can use 4K SD cards. I know that would only give you about 3 minutes of 4K, but 3 minutes was all you got out of a Super-8 cartridge, and lots of good stuff has been shot on that
I know an importer of SD cards who is willing to lend me some samples with a view to seeing if they are "RED-worthy" . Some of the Toshiba ones are marked "Ultra High Speed" , whatever that may mean.
Not having ready access to a RED One, I was wondering if there is some other way to determine the actual speed performance of an SD card.

Well, the whole problem with what you are talking about is that RED doesn't record onto SD cards--it uses Compact Flash cards. Also from my understanding, not just any CF cards will work. I believe starting in beta build 15 that RED stopped letting you use other cards that didn't ID as RED cards. I believe they did that because people were having so much trouble with third party cards that said they were 300x, but really were not. Not 100% positive on this info, but it seems like I did read something like it at RU.

Well, the whole problem with what you are talking about is that RED doesn't record onto SD cards--it uses Compact Flash cards. Also from my understanding, not just any CF cards will work. I believe starting in beta build 15 that RED stopped letting you use other cards that didn't ID as RED cards. I believe they did that because people were having so much trouble with third party cards that said they were 300x, but really were not. Not 100% positive on this info, but it seems like I did read something like it at RU.

Matthew

My Bad. I actually wrote "CF" when I was composing my original post, and then for some reason I got the idea I really meant "SD", and changed them all. All I remembered was that they were the type of cards that DON'T work in my still camera

So are you saying that RED will not allow you to use anything but the CF cards they supply?

My Bad. I actually wrote "CF" when I was composing my original post, and then for some reason I got the idea I really meant "SD", and changed them all. All I remembered was that they were the type of cards that DON'T work in my still camera

So are you saying that RED will not allow you to use anything but the CF cards they supply?

Kinda. There evidently were some Sony cards that worked in build 14 and before that now don't work in build 15. However, I found out yesterday that there are some Lexar cards (probably the same as the RED cards, RED just rebrands them) that you can use and after rebate are only $90! It seems that RED has found VERY few cards that are fast enough. It's a shame that RED couldn't have designed a small RAID-0 module out of two compact flash cards. That would have been VERY cool and they could have used slower cards.